1. Field of the Invention
The present invention relates generally to a CDMA (Code Division Multiple Access) mobile communication system, and in particular, to a method for enabling a base station to support a discontinuous transmission (DTX) mode in a dedicated control channel and a supplemental channel.
2. Description of the Related Art
Existing CDMA mobile communication systems have mainly provided voice service. However, in the near future, CDMA mobile communication systems will support the IMT-2000 (International Mobile Telecommunication-2000) standard which can provide data service as well as voice service. The IMT-2000 mobile communication system can support high-quality voice service, moving picture service and Internet search service.
A CDMA mobile communication system includes a base station (BS), which is comprised of a base station transceiver system (BTS) and a base station controller (BSC), a mobile switching center (MSC), and a mobile station (MS). A radio link existing between the MS and the BTS is divided into a forward link for transmitting a signal from the BTS to the MS and a reverse link for transmitting a signal from the MS to the BTS.
Every channel is divided into a physical channel and a logical channel. The logical channel is established over the physical channel, and several logical channels can be established on a single physical channel. If the physical channel is released, the logical channel established over the physical channel is automatically released. It is not necessary to establish the physical channel in order to establish a certain logical channel. When a physical channel to be established for a logical channel is already established for another logical channel, a required operation is only to assign this logical channel to the previously established physical channel.
The physical channel can be divided into a dedicated channel and a common channel according to its property. The dedicated channel is exclusively used for communication between the BTS and the MS, and includes a fundamental channel (FCH), a dedicated control channel (DCCH) and a supplemental channel (SCH). The fundamental channel is used to transmit voice signal, data signal and signaling signal. Such a fundamental channel is compatible with TIA/EIA-95-B. The dedicated control channel is used to transmit the data signal and signaling signal. The supplemental channel is used when large amounts of data need to be transmitted. The common channel is the physical channel other than the dedicated channel, and is commonly used by the base station and several mobile stations. A physical channel for the forward link transmitted from the BTS to the MS is called a paging channel, and a physical channel for the reverse link transmitted form the MS to the BTS is called an access channel. These common channels are compatible with IS-95-B.
In a mobile communication system, data communication has the characteristic that periods of bursty data transmission alternate with long periods of no data transmissions. Therefore, future mobile communication systems employ a discontinuous transmission (DTX) mode for assigning the dedicated channel only when data is transmitted during the data communication service.
The DTX mode refers to a mode in which a wired system or a mobile communication system transmits data on a frame unit basis only when there is data to transmit. That is, the DTX mode refers to a mode in which the wired system or the mobile communication system does not transmit data when there is no transmission data for a predetermined time period. The DTX mode has various advantages as follows. Since data is transmitted on a frame unit basis only when there is actual data, it is possible to minimize transmission power. Further, the overall interference of the system decreases in strength, thus increasing the overall system capacity.
However, since the frames are irregularly transmitted by the transmitter, the receiver cannot know beforehand whether frames have been transmitted or not. Accordingly, the BTS cannot independently perform forward power control. More specifically, if the receiver in the MS does not exactly know when the frame has been transmitted at the transmitter, the decision parameters of the decoder, including the cyclic redundancy code (CRC), and the decoding results are unreliable. Accordingly, in DTX mode, it is not possible to precisely control transmission power of the MS by applying the same method used in the continuous transmission mode.
The DTX mode is supported in the dedicated control channel and the supplemental channel. The dedicated control channel supports the DTX mode in which data is transmitted only when the upper layer generates transmission data. Because of such a property, the dedicated control channel is proper to be used as a control channel to effectively provide packet service. For this DTX period, it is possible to perform power control by transmitting a null frame over the dedicated control channel. The supplemental channel also supports the DTX mode for transmitting no data in a period where there is no data to transmit. In such a DTX period, no frame is transmitted over the supplemental channel. The DTX mode connects the dedicated traffic channel and control channel only in a period where the data is actually transmitted, and releases the dedicated channels when no data is transmitted for a predetermined time period, in consideration of the limited radio resources, the base station capacity, and the power consumption of the mobile station. When the dedicated channels are released, communication is performed through the common channel only, thereby increasing utilization efficiency of the radio resources. For such a DTX mode, there are required several states according to the channel assignment situation and existence/nonexistence of state information.
FIG. 1 shows a state transition diagram of a mobile communication system for a common packet service. Referring to FIG. 1, the states for the packet service are divided into an active state 11, a control hold state 12, a suspended state 13, a dormant state 14, a packet null state 15, and an initial state 10. In the control hold state 12, the active state 11, and suspended state 13, a service option is connected. In the other states, the service option is not connected. It should be noted that the present invention relates to the base station (BSC and BTS) for supporting the DTX mode in the supplemental channel and the dedicated channel in the active state 11 and the control hold state 12.
FIG. 2 shows a reference model of a 3G IOS (Interoperability Specifications) for a digital air interface between the MSC and the base station, and between the base stations in the common mobile communication system.
Referring to FIG. 2, between MSC 20 and BSC 32, a signal is defined as an A1 interface and user information is defined as A2/A5 (circuit data) interface. An A3 interface is defined to connect a target BS 40 to a frame selection/distribution unit (SDU) function block 34 of a source BS 30 for soft/softer handoff between the base stations. The signaling and user traffic between the target BS 40 and the SDU function block 34 of the source BS 30 are transmitted through the A3 interface. An A7 interface is defined for signal exchange between the target BS 40 and the source BS 30, for soft/softer handoff between the base stations. In a CDMA mobile communication system, the wired communication link between the base station 30 and the base station 40, and between the base station 30 and the MSC 20, is comprised of a forward link transmitted from the MSC 20 to the base station 30, a reverse link transmitted from the base station 30 to the MSC 20. Generally, a wired bi-directional line connected between the MSC 20 and the base station 30 carries the reverse and forward links. The wired line exists over all the interfaces in the RAN, e.g., MSC-BSC, BS-BS, BSC-BTS, BSC-Target BTS and so on. The MSC 20 includes a call control and mobility management block 22 and a switching block 24. Further, the MSC 20 is connected to a data network such as the Internet through an interworking function (IWF) block 50.
FIG. 3 shows a message format (hereinafter, referred to as FCH forward message (or data frame)) transmitted over a user traffic subchannel in the form of the fundamental channel (FCH) to the BTS 36 or 44 from the BSC 32 (or 42).
The message format shown in FIG. 3 is used to transmit a forward traffic channel frame to the base transmission system, and has information elements including a message type, forward layer-3 data, and a message CRC. This message is designed for sending the reverse/forward power control information from BSC/SDU to BTS. The reverse/forward power control information is necessary for the BTS to adjust the power control level of MS and the data frame, e.g., signaling message, packet frame data, and so on. The FCH forward message is a message used between a BSC and a BTS in the same base station, or a message used between a BSC and a BTS belonging to different base stations. The FCH forward message has a different name according to the corresponding interface. For example, a message transmitted between a BTS and BSC in the same BS is called an Abis FCH forward message, and a message transmitted between a BTS and a BSC belonging to the different BSs is called an A3 FCH forward message. FIG. 4 is a detailed diagram illustrating the information elements of the FCH forward message of FIG. 3.
Referring to FIG. 4, the forward layer-3 data portion of the FCH forward message includes CDMA forward traffic channel frame and control information for the packet transmitted to the target BTS 44 from the SDU function block 34 of FIG. 2. In FIG. 4, the power control information in this message, e.g., Forward Traffic Channel Gain, Reverse Traffic Channel Ew/Nt are used by the BTS to adjust the reverse/forward power control level for the given MS. The other control information in this message, e.g., Soft HO Leg #, Sequence Number, Rate Set indicator, Forward Traffic Channel Rate and Power Control Subchannel Count are used by the BTS to control the synchronization, the identification of the Soft Handoff Leg, and the knowledge for data rate information to be sent over the air between BSC-SDU and BTS. The forward layer-3 data has the structure shown in Table 1 below.
In Table 1, a first xe2x80x9cReservedxe2x80x9d field of octet 1 is set to xe2x80x980xe2x80x99 by the SDU function block. A xe2x80x9cSoft Handoff Leg #xe2x80x9d field is used to carry the soft handoff leg number as determined by the BS. A xe2x80x9cSequence Numberxe2x80x9d field is set to CDMA System Time in frames, modulo 16 (see 1.2 of TM/EIA-95) corresponding to the transmission time of the frame over the air in the forward direction. A xe2x80x9cForward Traffic Channel Gainxe2x80x9d field indicates the traffic channel gain obtained in the forward direction. A xe2x80x9cReverse Traffic Channel EW/NTxe2x80x9d field indicates traffic channel EW/NT required in the reverse direction. Here, EW denotes the total demodulated Walsh symbol energy and NT denotes the total received power spectral density on the RF channel. A xe2x80x9cRate Set Indicatorxe2x80x9d field indicates a Rate Set of the channel frame as shown in Table 2 below.
Referring to Table 2, the field value xe2x80x980000xe2x80x99 indicates the Rate Set 1, and the field value xe2x80x980001xe2x80x99 indicates the Rate Set 2.
A xe2x80x9cForward Traffic Channel Ratexe2x80x9d field of Table 1 indicates the rate at which the BTS transmits the forward traffic channel information to the MS, and will be set as shown in Table 3 below.
As shown in Table 3, for the field value xe2x80x980000xe2x80x99, the forward traffic channel information is transmitted at the full rate; for the field value xe2x80x980001xe2x80x99, the forward traffic channel information is transmitted at the half (xc2xd) rate; for the field value xe2x80x980010xe2x80x99, the forward traffic channel information is transmitted at the quarter (xc2xc) rate; and for the field value xe2x80x980011xe2x80x99, the forward channel information is transmitted at the eighth (xe2x85x9) rate. If the field value is xe2x80x980100xe2x80x99, an idle frame is transmitted. For an idle frame, the BTS does not transmit the frame and ignores all the information elements other than the Sequence Number field and the Frame Type field. Such an idle frame is used to adjust the frame arrival time.
A second xe2x80x9cReservedxe2x80x9d field of octet 5 in Table 1 is set to xe2x80x980000xe2x80x99. A xe2x80x9cPower Controll Subchannel Countxe2x80x9d field indicates the number of independent power control subchannels involved in soft handoff. A xe2x80x9cForward Traffic Channel Informationxe2x80x9d field indicates the forward traffic channel information that the BTS is to send to the MS. The transmission rate can be any one of the transmission rates shown in Table 4 below. A xe2x80x9cLayer-3 Fillxe2x80x9d field indicates the number of bits in the Layer-3 Fill column corresponding to the transmission rate of the forward traffic, and can be any one of those shown in Table 5 below.
FIG. 5 shows a message format (hereinafter, referred to as FCH reverse message (or data frame)) transmitted over a user traffic subchannel in the form of the fundamental channel (FCH) to the BSC 32 (or 42) from the BTS 36 (or 44) of FIG. 2.
The message format show in FIG. 5 is used to transmit the decoded reverse traffic channel frame and control information in the BTS, and has an information elements including a message type II, reverse layer-3 data and a message CRC. The FCH reverse message is a message used between a BSC and a BTS belonging to the same BS, or a message used between a BTS and a BSC belonging to different BSs. The FCH forward message has a different name according to the corresponding interface. For example, a message transmitted from a BTS to a BSC belonging to the same BS is called an Abis FCH reverse message, and a message transmitted between a BTS and a BSC belonging to different BSs is called an A3 FCH reverse message.
FIG. 6 is a detailed diagram illustrating the information element of the FCH reverse message of FIG. 5.
Referring to FIG. 6, the reverse layer-3 portion data of the FCH reverse message includes CDMA reverse traffic channel frame and control information for the packet transmitted to the SDU function block from the target BTS. In FIG. 6, the power control information in this message, e.g., Reverse Traffic Channel Quality, EIB are used by the BSC/SDU to determine the reverse/forward power control level to be sent to the BTS. The other control information in this message, e.g., Soft HO Leg #, Sequence Number, Rate Set indicator, Reverse Traffic Channel Rate, Scaling, and Packet Arrival Time Error, are used by the BSC/SDU to control the timing for sending the forward Layer 3 data in FIG. 3 or 4, the identification of the Soft Handoff Leg and the knowledge for data rate information to be received over the air. This communication message is mainly BTS to BSC/SDU, i.e., source BTS to source BSC/SDU and target BTS to source BSC/SDU. The reverse layer-3 data has the structure shown in Table 6 below.
In Table 6, a first xe2x80x9cReservedxe2x80x9d field of octet 1 is set to xe2x80x980xe2x80x99 by the BTS. A xe2x80x9cSoft Handoff Leg #xe2x80x9d field is used to carry the soft handoff leg number as determined by the source BS on the A3-FCH forward message. A xe2x80x9cSequence Numberxe2x80x9d field is set to CDMA System Time in frames, modulo 16 (see 1.2 of TIA/EIA-95) corresponding to the receiving time of the air interface frame in the reverse direction. A xe2x80x9cReverse Traffic Channel Qualityxe2x80x9d field consists of a 1-bit CRC field and a 7-bit symbol error rate field. The 7-bit symbol error rate is the binary value of
127xe2x88x92(Min[Re-Encoded Symbol Error Ratexc3x97xcex1, 255])/2
where the value of a is determined according to the reverse traffic channel rate as shown in Table 7 below.
As shown in Table 7, for the full rate, the value xcex1 is 1; for the half (xc2xd) rate, the value xcex1 is 2; for the quarter (xc2xc) rate, the value xcex1 is 4; and for the eighth (xc2xd) rate, the value xcex1 is 8. If the most recently received forward frame received by the BTS from the SDU function block was an idle frame, then the BTS shall set the xe2x80x9cReverse Traffic Channel Qualityxe2x80x9d field to a value of 00H and shall send an idle frame to the SDU function block. The SDU function block shall ignore the value of this field in idle frames.
In Table 6, a xe2x80x9cScalingxe2x80x9d field is the time scale for the xe2x80x9cPacket Arrival Time Error (PATE)xe2x80x9d field. The xe2x80x9cPacket Arrival Timer Errorxe2x80x9d field indicates a time difference between the time at which the A3-FCH Forward message arrives and an average arrival time measured in units specified by the xe2x80x9cScalingxe2x80x9d field, and can have the field values shown in Table 8 below.
A xe2x80x9cRate Set Indicatorxe2x80x9d field of Table 6 indicates a rate set of the traffic channel frame. If the BTS is sending an idle frame to the SDU function block, the SDU function block shall ignore the contents of this field. As shown in Table 9below, the xe2x80x9cRate Set Indicatorxe2x80x9d field value of xe2x80x980000xe2x80x99 indicates the Rate Set 1, and the field value xe2x80x980001xe2x80x99 indicates the Rate Set 2.
A xe2x80x9cReverse Traffic Channel Ratexe2x80x9d field of Table 6 indicates a transmission rate for the traffic channel information transmitted from the MS to the BTS, i.e., a transmission rate for sending the reverse traffic channel information, and can have the field values shown in Table 10 below. The field value xe2x80x980000xe2x80x99 corresponds to the full rate, the field value xe2x80x980001xe2x80x99 corresponds to the half (xc2xd) rate, the field value xe2x80x980010xe2x80x99 corresponds to the quarter (xc2xc) rate, and the field value xe2x80x980011xe2x80x99 corresponds to the eighth (xe2x85x9) rate. If the BTS did not acquire the MS, the BTS defines the reverse traffic channel rate information having the field value xe2x80x980101xe2x80x99 as idle.
A xe2x80x9cReverse Traffic Channel Informationxe2x80x9d field indicates reverse traffic channel information that the BTS has received from the MS. The xe2x80x9cReverse Traffic Channel Informationxe2x80x9d field includes the number of information bits per frame, shown in Table 11 below, according to the rate sets. For example, for the Rate Set 1, when the transmission rate is 9600 bps, the number of information bits per frame is 172; and when the transmission rate is 1200 bps, the number of information bits per frame is 16. For the Rate Set 2, when the transmission rate is 14400 bps, the number of information bits per frame is 267; and when the transmission rate is 3600 bps, the number of information bits per frame is 55.
A xe2x80x9cEIB (Erasure Indication Bit)xe2x80x9d field of Table 6 indicates that an erasure frame has been transmitted. When Rate Set 1 is being used, the BTS shall set this bit to xe2x80x980xe2x80x99. When Rate Set 2 is being used, the BTS shall set this bit to xe2x80x981xe2x80x99. A second xe2x80x9cReservedxe2x80x9d field of octet 5 is set to xe2x80x980000000xe2x80x99. A xe2x80x9cLayer-3 Fillxe2x80x9d field indicates the number of bits in the Layer-3 Fill column corresponding to the transmission rate of the reverse traffic channel frame, and can be any one of those shown in Table 12 below according to the Rate Sets.
FIGS. 7 and 8 show soft/softer handoff addition and removal procedures, respectively, according to the prior art. These procedures are performed on the conventional FCH frame.
First, the soft/softer handoff addition procedure will be described with reference to FIG. 7.
In step 7a, the source BS 30 of FIG. 2 decides that one or more cells of the target BS 40 are required to support a present call in service during soft handoff, sends an A7-handoff request message to the target BS 40 and then activates a timer Thoreq. In step 7b, the target BS 40 initiates A3 connection by sending an A3-connect message to a designated address for A3 connection required by the A7-handoff request message. In step 7c, the source BS 30 sends an A3-connect Ack message to acknowledge completion of A3 connection or addition of the cells to the existing A3 connection. In step 7d, the source BS 30 starts to transmit the forward frame to the target BS 40.
As synchronized with the source BS 30, the target BS 40 starts to transmit the forward frame to the MS in step 7e. Upon receipt of the first forward frame from the source BS 30, the target BS 40 starts to transmit a reverse idle frame to the source BS 30 in step 7f. The transmitted reverse idle frame includes time control information required for acquiring synchronization. The target BS 40 sends an A7-handoff request Ack message indicating success in cell addition, in step 7g. The source BS 30 then inactivates the timer Thoreq in response to the A7-handoff request Ack message. If the source BS 30 is selected such that the source BS 30 is to know transmission start and acceptance of the target BS 40 when the SDU function block 34 of the source BS 30 and the target BS 40 synchronize the A3-traffic subchannel, the target BS 40 sends an A3-traffic channel status message in step 7h. The step 7h is performed after the step 7d. 
In step 7i, the source BS 30 sends a handoff direction message to the MS to add new cells to an active set. In step 7j, the MS sends an MS Ack order message indicating acknowledgement of the handoff direction message to the source BS 30. In step 7k, the MS sends a handoff completion message to the source BS 30 to notify successful process of the handoff direction message. In step 71, the source BS 30 sends a BS Ack order message to the MS to acknowledge receipt of the handoff completion message. In step 7m, the source BS 30 sends a handoff performed message to the MSC. The handoff performed message can be transmitted any time after the source BS 30 receives the handoff completion message.
A Next, the soft/softer handoff removal procedure will be described with reference to FIG. 8.
In step 8a, the source BS 30 encapsulates a handoff direction message in an A3-FCH forward message and transmits it to the target BS 40 to drop one or more cells from the active set. In step 8b, the source BS 30 and the target BS 40 send the handoff direction message to the MS. In step 8c, the MS sends an MS Ack order message to both the source BS 30 and the target BS 40 to acknowledge receipt of the handoff direction message. In step 8d, the target BS 40 sends the MS Ack order message received from the MS to the source BS 30 by loading the MS Ack order message in an A3-FCH reverse message. In step 8e, the MS sends a handoff completion message to the source BS 30 to indicate successful processing of the handoff direction message. In step 8f, the source BS 30 sends a BS Ack order message to the MS to acknowledge receipt of the handoff completion message.
The prior art has the following problems occurring in the base station, rather than in a radio link between the base station and the mobile station.
1) Absence of DCCH related supporting method and device
As shown in FIGS. 3 to 8, a method and device for processing the dedicated control channel (DCCH) newly added in the CDMA-2000 system is never defined in the existing standard. Therefore, a frame transmitted over the forward and reverse DCCHs between the BSC and the BTS is not defined and how to perform power control through the DCCH while the DCCH is in use, is also not defined.
2) Absence of DTX mode supporting method
A method for processing the SCH and DCCH in a DTX mode, which does not exist in the existing FCH, is not defined. For example, as shown in FIGS. 7 and 8, in the existing 3G IOS, the soft/softer handoff procedure and message is not defined on the DCCH frame. In addition, unlike the FCH, the DCCH supports the DTX mode in which no frame is generated and transmitted when there is no data, signaling, power control and MAC (Medium Access Control) signals transmitted from the upper layer to the physical layer. Since the message and procedure for the DCCH frame and the DTX mode operation is not presently defined on the soft/softer handoff-related A3 and A7 interfaces, there is required a message to be bi-directionally transmitted at the A3 and A7 interfaces for the DCCH frame, and a procedure for controlling and supporting the DTX mode on the DCCH in the base station.
It is, therefore, an object of the present invention to provide a method for transmitting and receiving a newly defined forward and reverse frame (message) over a dedicated control channel (DCCH) between a base station controller (BSC) and a base station transceiver system (BTS) in a mobile communication system.
It is another object of the present invention to provide a method for determining power control information and transmitting and receiving the power control information over a dedicated control channel between the base station controller and the base station transceiver system in a mobile communication system.
It is further another object of the present invention to provide a method for transmitting and receiving a signal over the dedicated control channel between the base station controller and the base station transceiver system during the discontinuous transmission (DTX) mode, in which data is transmitted only in a period where there is data to transmit, in a mobile communication system.
It is yet another object of the present invention to provide a method for determining power control information and transmitting and receiving the power control information over the dedicated control channel between the base station controller and the base station transceiver system during the discontinuous transmission mode, in which data is transmitted only in a period where there is data to transmit, in a mobile communication system.
It is still another object of the present invention to provide a soft/softer handoff method performed over the dedicated control channel in a mobile communication system.
It is still another object of the present invention to provide a soft/softer handoff method performed over the dedicated control channel during the discontinuous transmission mode, in which data is only transmitted in a period where there is data to transmit, in a mobile communication system.
To achieve the above objects, there is provided a method for transmitting a signal from a base station transceiver system (BTS) to a base station controller (BSC) when there is no data transmitted from a mobile station while in a discontinuous transmission (DTX) mode in a mobile communication system. The mobile communication system includes a mobile station for transmitting/receiving data in a predetermined period, a base station transceiver system and a base station controller for controlling the base station transceiver system. Upon detection of the discontinuous transmission mode, the base station transceiver system sets previous power control information that the base station transceiver system has used for power control of the mobile station before detection of the discontinuous transmission mode, to present power control information to the base station controller. Thereafter, the base station transceiver system transmits a reverse message including the present power control information to the base station controller over a dedicated control channel. When there is data being transmitted between the mobile station and the base station transceiver system at the time when the discontinuous transmission mode is detected, the previous power control information is set to the present power control information.